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1 May 2012

Volume 111, Issue 9, Articles (09xxxx)

Issue Cover Spotlight Figure

J. Appl. Phys. 111, 093103 (2012); http://dx.doi.org/10.1063/1.4709385 (8 pages)

Ani Khachatrian, Joseph S. Melinger, and Syed B. Qadri
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back to top Structural, Mechanical, Thermodynamic, and Optical Properties of Condensed Matter

Spectroscopic ellipsometry model for optical constant of NiSi formed on silicon-on-insulator substrates

A. Vellei, R. Fallica, D. Sangalli, and A. Lamperti

J. Appl. Phys. 111, 093501 (2012); http://dx.doi.org/10.1063/1.4706561 (6 pages)

Online Publication Date: 1 May 2012

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Nickel silicide is considered the best candidate material to achieve the lowest contact resistance in sub 45 nm CMOS devices. NiSi films with thickness 20–60 nm were prepared by rapid thermal annealing of Ni (temperature 230 °C–780 °C) on top of thin 230 nm silicon-on-insulator substrates, with a constant formation ratio. Based on film independent characterizations, a novel model for the interpretation of spectroscopic ellipsometry data, featuring a combination of two Lorentzian oscillators and one Drude dispersion model, is proposed, and its goodness is checked in comparison to other known models. This new approach is proved to deliver more accurate estimation of the film thickness and resistivity.
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78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
68.55.jd Thickness
61.72.Cc Kinetics of defect formation and annealing
73.40.Cg Contact resistance, contact potential

Time-resolved mirage method: A three-dimensional theory and experiments

N. G. C. Astrath, L. C. Malacarne, H. S. Bernabe, M. L. Baesso, and C. Jacinto

J. Appl. Phys. 111, 093502 (2012); http://dx.doi.org/10.1063/1.4707940 (6 pages) | Cited 1 time

Online Publication Date: 1 May 2012

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A general time-resolved three-dimensional theory of the photothermal beam deflection for the measurement of thermal properties of opaque materials is presented. We derive the analytical solutions for the laser induced temperature profiles in the sample and in the fluid above the sample assuming flux discontinuity at the interface sample/fluid. We compare the analytical solutions with all numerical modeling using finite element analysis. The photothermal deflection signal is calculated and an expression is provided for the transverse photothermal signal at a position-sensing detector. We use the model and the experimental method to investigate opaque plastic and metals, and the results for the thermal properties of the samples are in an excellent agreement in the literature values.
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65.90.+i Other topics in thermal properties of condensed matter (restricted to new topics in section 65)
81.40.Lm Deformation, plasticity, and creep
62.20.fq Plasticity and superplasticity
02.60.-x Numerical approximation and analysis
42.62.-b Laser applications

Reversible high-pressure phase transition in LaN

Sebastian B. Schneider, Dominik Baumann, Ashkan Salamat, and Wolfgang Schnick

J. Appl. Phys. 111, 093503 (2012); http://dx.doi.org/10.1063/1.4709392 (6 pages) | Cited 3 times

Online Publication Date: 1 May 2012

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In situ high-pressure X-ray powder diffraction experiments on LaN up to 60.1 GPa at ambient temperature in a diamond-anvil cell revealed a reversible, first-order structural phase transition starting at ∼22.8 GPa and completed at ∼26.5 GPa from the ambient cubic phase (Fmmathm, no. 225) to a tetragonal high-pressure phase (P4/nmm, no. 19, a = 4.1060(6), c = 3.0446(6) Å, Z = 2, wRp = 0.011), which has not been claimed in theoretical predictions. HP-LaN is isotypic with a high-pressure polymorph of BaO, which crystallizes in a tetragonally distorted CsCl-type structure. The phase transition is accompanied by a volume collapse of about 11% which corresponds well with the reported data on HP-BaO. A linear extrapolation of the c/a ratio of the tetragonally distorted CsCl-type sub-cell reaches a value c/a = 1 of cubic CsCl-type HP-LaN at 91(12) GPa. In addition, the compressibility of LaN was investigated and resulted in a bulk modulus for the ambient pressure phase of B0 = 135(3) GPa and B′ = 5.0(5) after fitting a third-order Birch-Murnaghan equation of state to the experimental p–V data. The corresponding extrapolated bulk modulus of HP-LaN is found to be B0 = 278(6) GPa and its pressure derivative B′ = 1.2(2). Both as-calculated bulk moduli are compared to the respective values obtained from an Eulerian strain versus normalized stress plot to be 143(2) GPa for ambient LaN and 293(7) GPa for HP-LaN. Compared to other binary nitrides such as δ-ZrN or δ-HfN having bulk moduli of 285 GPa and 306 GPa, respectively, the extrapolated bulk moduli of HP-LaN are in the same order of magnitude, ranking HP-LaN as a highly incompressible material.
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61.50.Ks Crystallographic aspects of phase transformations; pressure effects
62.50.-p High-pressure effects in solids and liquids
64.70.kp Ionic crystals
61.66.Fn Inorganic compounds
81.40.Lm Deformation, plasticity, and creep
81.40.Jj Elasticity and anelasticity, stress-strain relations

Buckled colloidal crystals with nonspherical bases for two-dimensional slab photonic band gaps

E. K. Riley, E. Y. Fung, and C. M. Liddell Watson

J. Appl. Phys. 111, 093504 (2012); http://dx.doi.org/10.1063/1.4706556 (9 pages)

Online Publication Date: 2 May 2012

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Theoretical modeling of the photonic band gap forming properties is reported for the buckled phase of anisotropic particles. These exist between the first and second particulate layers of confined colloidal suspensions. Inspired by the range of non-spherical mushroom-cap building blocks for self-assembly that have been synthesized using seeded emulsion-polymerization, we explore in particular the band structures as a function of toroid shape parameter. The parameter is adjusted to incrementally transform hemispheres to spheres. Additionally, corrugation heights that systematically modulate the slab photonic crystal unit cell from rectangular monolayer to square bilayer are investigated. Polarization independent gaps in the guided modes are determined for direct and inverted structures that exhibit bifurcation in the particle orientation perpendicular to the slab plane. Gaps in the guided modes are observed between the fourth and fifth, twelfth and thirteenth, as well as higher band locales as the particle morphology and lattice aspect ratio vary.
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82.70.Dd Colloids
82.70.Kj Emulsions and suspensions
87.15.rp Polymerization
42.70.Qs Photonic bandgap materials
62.90.+k Other topics in mechanical and acoustical properties of condensed matter (restricted to new topics in section 62)
78.67.Pt Multilayers; superlattices; photonic structures; metamaterials

Designing interlayers to improve the mechanical reliability of transparent conductive oxide coatings on flexible substrates

Eun-Hye Kim, Chan-Woo Yang, and Jin-Woo Park

J. Appl. Phys. 111, 093505 (2012); http://dx.doi.org/10.1063/1.4709295 (8 pages)

Online Publication Date: 2 May 2012

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In this study, we investigate the effect of interlayers on the mechanical properties of transparent conductive oxide (TCO) on flexible polymer substrates. Indium tin oxide (ITO), which is the most widely used TCO film, and Ti, which is the most widely used adhesive interlayer, are selected as the coating and the interlayer, respectively. These films are deposited on the polymer substrates using dc-magnetron sputtering to achieve varying thicknesses. The changes in the following critical factors for film cracking and delamination are analyzed: the internal stress (σi) induced in the coatings during deposition using a white light interferometer, the crystallinity using a transmission electron microscope, and the surface roughness of ITO caused by the interlayer using an atomic force microscope. The resistances to the cracking and delamination of ITO are evaluated using a fragmentation test. Our tests and analyses reveal the important role of the interlayers, which significantly reduce the compressive σi that is induced in the ITO and increase the resistance to the buckling delamination of the ITO. However, the relaxation of σi is not beneficial to cracking because there is less compensation for the external tension as σi further decreases. Based on these results, the microstructural control is revealed as a more influential factor than σi for improving crack resistance.
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81.40.Np Fatigue, corrosion fatigue, embrittlement, cracking, fracture, and failure
81.40.Lm Deformation, plasticity, and creep
81.15.Cd Deposition by sputtering
62.20.mq Buckling
62.20.mt Cracks
68.35.B- Structure of clean surfaces (and surface reconstruction)

Electronic and optical properties of free-standing and supported vanadium nanowires

Poorva Singh, Tashi Nautiyal, and Sushil Auluck

J. Appl. Phys. 111, 093506 (2012); http://dx.doi.org/10.1063/1.4709432 (9 pages) | Cited 2 times

Online Publication Date: 2 May 2012

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We have investigated theoretically the electronic and optical properties of free-standing and substrate-supported ultrathin nanowires (NWs) of the transition metal vanadium. Ground state of the structures studied, except free-standing zigzag geometry, is found to be magnetic in nature. We show that for some structures, study of the antiferromagnetic state necessitates considering various possible configurations. All the structures, except dimerized, show metallic behavior. Structure with helical geometry possesses decent value of magnetic moment and is exceptionally stable as well as most stiff of all the structures studied. The plasma frequency and dielectric function nicely exhibit the anisotropy due to one-dimensional nature of the nanowires. The latter is structure-dependent and markedly different from that of bulk. More realistic case of linear chains supported on a substrate shows fair impact of the substrate in comparison with free-standing case. There is substantial charge redistribution on relaxing the geometry. The d-states are in general shifted to lower energies and the peaks in the density of states are broadened, resulting in softening of the structures in the optical spectra.
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73.22.-f Electronic structure of nanoscale materials and related systems
78.67.Uh Nanowires
75.25.-j Spin arrangements in magnetically ordered materials (including neutron and spin-polarized electron studies, synchrotron-source x-ray scattering, etc.)
75.50.Ee Antiferromagnetics
75.30.Cr Saturation moments and magnetic susceptibilities
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)

Excitation power and temperature dependence of excitons in CuInSe2

F. Luckert, M. V. Yakushev, C. Faugeras, A. V. Karotki, A. V. Mudryi, and R. W. Martin

J. Appl. Phys. 111, 093507 (2012); http://dx.doi.org/10.1063/1.4709448 (8 pages) | Cited 2 times

Online Publication Date: 2 May 2012

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Excitonic recombination processes in high quality CuInSe2 single crystals have been studied by photoluminescence (PL) and reflectance spectroscopy as a function of excitation powers and temperature. Excitation power dependent measurements confirm the identification of well-resolved A and B free excitons in the PL spectra and analysis of the temperature quenching of these lines provides values for activation energies. These are found to vary from sample to sample, with values of 12.5 and 18.4 meV for the A and B excitons, respectively, in the one showing the highest quality spectra. Analysis of the temperature and power dependent PL spectra from the bound excitonic lines, labelled M1, M2, and M3 appearing in multiplets points to a likely assignment of the hole involved in each case. The M1 excitons appear to involve a conduction band electron and a hole from the B valence band hole. In contrast, an A valence band hole appears to be involved for the M2 and M3 excitons. In addition, the M1 exciton multiplet seems to be due to the radiative recombination of excitons bound to shallow hydrogenic defects, whereas the excitons involved in M2 and M3 are bound to more complex defects. In contrast to the M1 exciton multiplet, the excitonic lines of M2 and M3 saturate at high excitation powers suggesting that the concentration of the defects involved is low.
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71.35.-y Excitons and related phenomena
71.20.Nr Semiconductor compounds
78.55.Hx Other solid inorganic materials

Ultrahigh electromechanical response in (1−x)(Na0.5Bi0.5)TiO3-xBaTiO3 single-crystals via polarization extension

Wenwei Ge, Chengtao Luo, Qinhui Zhang, Chris P. Devreugd, Yang Ren, Jiefang Li, Haosu Luo, and D. Viehland

J. Appl. Phys. 111, 093508 (2012); http://dx.doi.org/10.1063/1.4709619 (8 pages) | Cited 3 times

Online Publication Date: 2 May 2012

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The dielectric, ferroelectric, and electric field-induced strain response of [001]-and [101]-oriented 0.944Na0.5Bi0.5TiO3-0.056BaTiO3 (0.944NBT-0.056BT) single crystals were investigated as a function of temperature and dc bias (E). An ultrahigh electromechanical response with large amplitude longitudinal piezoelectric coefficients as high as d33 = 2500 pm/V was found in [001]PC oriented 0.944NBT-0.056BT single crystals near a depolarization temperature of Td = 130 °C. In-situ XRD revealed that the enhanced piezoelectric properties resulted from a polarization extension between a polar pseudocubic phase with a slight tetragonal (P4bm) distortion and a polar tetragonal one with a large tetragonal distortion of c/a = 1.02. Our findings indicate a potential approach to high performance lead-free piezoelectrics, via polarization extension.
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77.22.Ej Polarization and depolarization
77.65.-j Piezoelectricity and electromechanical effects
77.80.-e Ferroelectricity and antiferroelectricity

Intrinsic stress in ZrN thin films: Evaluation of grain boundary contribution from in situ wafer curvature and ex situ x-ray diffraction techniques

L. E. Koutsokeras and G. Abadias

J. Appl. Phys. 111, 093509 (2012); http://dx.doi.org/10.1063/1.4710530 (8 pages) | Cited 2 times

Online Publication Date: 3 May 2012

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Low-mobility materials, like transition metal nitrides, usually undergo large residual stress when sputter-deposited as thin films. While the origin of stress development has been an active area of research for high-mobility materials, atomistic processes are less understood for low-mobility systems. In the present work, the contribution of grain boundary to intrinsic stress in reactively magnetron-sputtered ZrN films is evaluated by combining in situ wafer curvature measurements, providing information on the overall biaxial stress, and ex situ x-ray diffraction, giving information on elastic strain (and related stress) inside crystallites. The thermal stress contribution was also determined from the in situ stress evolution during cooling down, after deposition was stopped. The stress data are correlated with variations in film microstructure and growth energetics, in the 0.13–0.42 Pa working pressure range investigated, and discussed based on existing stress models. At low pressure (high energetic bombardment conditions), a large compressive stress is observed due to atomic peening, which induces defects inside crystallites but also promotes incorporation of excess atoms in the grain boundary. Above 0.3–0.4 Pa, the adatom surface mobility is reduced, leading to the build-up of tensile stress resulting from attractive forces between under-dense neighbouring column boundary and possible void formation, while crystallites can still remain under compressive stress.
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68.60.Bs Mechanical and acoustical properties
81.15.Cd Deposition by sputtering
81.40.Jj Elasticity and anelasticity, stress-strain relations
62.20.dq Other elastic constants
68.55.aj Insulators
61.72.Mm Grain and twin boundaries

An analytical study of the pulsed thermography defect detection limit

D. P. Almond and S. G. Pickering

J. Appl. Phys. 111, 093510 (2012); http://dx.doi.org/10.1063/1.4704684 (9 pages) | Cited 2 times

Online Publication Date: 3 May 2012

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A simple modification of the one-dimensional expression for the thermal contrast of a layer provides a useful prediction of peak contrast temperature and contrast peak time for defects of all aspect ratios. The new analytical results have been shown to agree with numerical modelling. The thermographic nondestructive evaluation (NDE) rule-of-thumb that defects are detected if aspect ratio exceeds two is shown to have no general validity as peak contrast is found to depend critically on defect depth and absorbed excitation energy as well as defect aspect ratio. The effects of thermal diffusivity anisotropy are included in the analysis and illustrated by simulations of defect image contrast in composite materials.
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81.70.Fy Nondestructive testing: optical methods
66.30.Xj Thermal diffusivity

Studying femtosecond-laser hyperdoping by controlling surface morphology

Mark T. Winkler, Meng-Ju Sher, Yu-Ting Lin, Matthew J. Smith, Haifei Zhang, Silvija Gradečak, and Eric Mazur

J. Appl. Phys. 111, 093511 (2012); http://dx.doi.org/10.1063/1.4709752 (7 pages) | Cited 3 times

Online Publication Date: 4 May 2012

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We study the fundamental properties of femtosecond-laser (fs-laser) hyperdoping by developing techniques to control the surface morphology following laser irradiation. By decoupling the formation of surface roughness from the doping process, we study the structural and electronic properties of fs-laser doped silicon. These experiments are a necessary step toward developing predictive models of the doping process. We use a single fs-laser pulse to dope silicon with sulfur, enabling quantitative secondary ion mass spectrometry, transmission electron microscopy, and Hall effect measurements. These measurements indicate that at laser fluences at or above 4 kJ m−2, a single laser pulse yields a sulfur dose >(3 ± 1) × 1013 cm−2 and results in a 45-nm thick amorphous surface layer. Based on these results, we demonstrate a method for hyperdoping large areas of silicon without producing the surface roughness.
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61.72.uf Ge and Si
68.47.Fg Semiconductor surfaces
79.20.Ds Laser-beam impact phenomena
79.20.Rf Atomic, molecular, and ion beam impact and interactions with surfaces
82.53.Mj Femtosecond probing of semiconductor nanostructures
82.80.Ms Mass spectrometry (including SIMS, multiphoton ionization and resonance ionization mass spectrometry, MALDI)

Optical properties of armchair graphene nanoribbons embedded in hexagonal boron nitride lattices

Hamed Nematian, Mahdi Moradinasab, Mahdi Pourfath, Morteza Fathipour, and Hans Kosina

J. Appl. Phys. 111, 093512 (2012); http://dx.doi.org/10.1063/1.4710988 (6 pages)

Online Publication Date: 4 May 2012

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Transition rules and optical properties of armchair graphene nanoribbons embedded in hexagonal boron nitride lattices are studied for the first time. Based on tight binding calculations considering first and second nearest neighbors, we show that the optical transition rules of such structures are completely different from that of conventional graphene nanoribbons. These rules are explained by the symmetry properties of the subband wave functions. The optical spectrum, the quantum efficiency, and the photoresponsivity of different nanoribbons are evaluated and their application in photodetector devices is investigated. The results are verified with first principles calculations.
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85.60.Gz Photodetectors (including infrared and CCD detectors)
07.57.Kp Bolometers; infrared, submillimeter wave, microwave, and radiowave receivers and detectors

Porosity-induced relaxation of strains in GaN layers studied by means of micro-indentation and optical spectroscopy

Adel Najar, Michel Gerland, and Mustapha Jouiad

J. Appl. Phys. 111, 093513 (2012); http://dx.doi.org/10.1063/1.4710994 (5 pages)

Online Publication Date: 4 May 2012

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We report the fabrication of porous GaN nanostructures using UV-assisted electroless etching of bulk GaN layer grown on c-plane sapphire substrate in a solution consisting of HF:CH3OH:H2O2. The morphology of the porous GaN nanostructures was characterized for different etching intervals using high resolution scanning electron microscopy. The geometry and size of resultant pores do not appear to be affected by the etching time; however, the pore density was augmented for longer etching time. Micro-indentation tests were carried out to quantify the indentation modulus for different porous GaN nanostructures. Our results reveal a relationship between the elastic properties and the porosity kinetics, i.e., a decrease of the elastic modulus was observed with increasing porosity. The photoluminescence (PL) and Raman measurements carried out at room temperature for the etched samples having a high degree of porosity revealed a strong enhancement in intensity. Also, the peak of the PL wavelength was shifted towards a lower energy. The high intensity of PL was correlated to an increase of scattered photons within the porous media and to the reduction of the dislocation density.
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81.16.Rf Micro- and nanoscale pattern formation
81.40.Jj Elasticity and anelasticity, stress-strain relations
81.40.Np Fatigue, corrosion fatigue, embrittlement, cracking, fracture, and failure
78.67.Rb Nanoporous materials
78.55.Cr III-V semiconductors
81.05.Ea III-V semiconductors

Te-based chalcogenide films with high thermal stability for phase change memory

Guoxiang Wang, Xiang Shen, Qiuhua Nie, Fen Chen, Xunsi Wang, Jing Fu, Yu Chen, Tiefeng Xu, Shixun Dai, Wei Zhang, and Rongping Wang

J. Appl. Phys. 111, 093514 (2012); http://dx.doi.org/10.1063/1.4711069 (5 pages) | Cited 2 times

Online Publication Date: 4 May 2012

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This study reports on the synthesis of tellurium-based chalcogenide films that have high thermal stability for phase change memory application. Several Te-based chalcogenide alloys of In-Bi-Te, Ag-Bi-Te, In-Sb-Te, Sn-Sb-Te, Zn-Ge-Te, and Ga-Ge-Te are reported. Their thermal, optical, and electrical properties are investigated. The results show that Bi-Te-based films have a higher crystallization temperature and greater activation energy compared with the other Sb-Te-based and Ge-Te-based films. Especially, In2.8Bi36.6Te60.6 film exhibits high crystallization temperature (252 °C) and great activation energy (5.16 eV), showing much improved amorphous thermal stability. A relatively wider optical band gap (0.674 eV) of thermal annealed In2.8Bi36.6Te60.6 film is obtained. In addition, it also has a higher amorphous/crystalline resistance ratio of about 105, implying that current consumption could be low in the phase-change memory operation.
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68.55.ag Semiconductors
73.61.Jc Amorphous semiconductors; glasses
78.66.Jg Amorphous semiconductors; glasses
68.60.Dv Thermal stability; thermal effects
71.20.Nr Semiconductor compounds

Precision equation-of-state measurements on National Ignition Facility ablator materials from 1 to 12 Mbar using laser-driven shock waves

M. A. Barrios, T. R. Boehly, D. G. Hicks, D. E. Fratanduono, J. H. Eggert, G. W. Collins, and D. D. Meyerhofer

J. Appl. Phys. 111, 093515 (2012); http://dx.doi.org/10.1063/1.4712050 (9 pages) | Cited 2 times

Online Publication Date: 7 May 2012

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A large uncertainty in the design of ignition capsules for use in the National Ignition Campaign (NIC) is the ablator equation of state. In this article, we report equation-of-state measurements for two candidate NIC ablator materials, glow-discharge polymer (GDP), and germanium-doped GDP. These materials were driven to pressures of 1 to 12 Mbar using laser-driven shock waves. Hugoniot measurements were obtained using the impedance matching technique with an α-quartz standard. This article presents the first kinematic measurements in the high-pressure fluid regime for these materials, which show to be in close agreement with Livermore equation-of-state model predictions.
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64.30.Jk Equations of state of nonmetals
62.50.Ef Shock wave effects in solids and liquids

Metastable phase formation in the Au-Si system via ultrafast nanocalorimetry

M. Zhang, J. G. Wen, M. Y. Efremov, E. A. Olson, Z. S. Zhang, L. Hu, L. P. de la Rama, R. Kummamuru, K. L. Kavanagh, Z. Ma, and L. H. Allen

J. Appl. Phys. 111, 093516 (2012); http://dx.doi.org/10.1063/1.4712342 (7 pages)

Online Publication Date: 7 May 2012

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We have investigated the stability and solidification of nanometer size Au-Si droplets using an ultrafast heating/cooling nanocalorimetry and in situ growth techniques. The liquid can be supercooled to very low temperatures for both Au-rich (ΔT ∼ 95 K) and Si-rich (ΔT ∼ 220 K) samples. Solidification of a unique metastable phase δ1 is observed with a composition of 74 ± 4 at. % Au and a b-centered orthorhombic structure (a = 0.92, b = 0.72, and c = 1.35 nm; body-center in the a-c plane), which grows heteroepitaxially to Aus. Its melting temperature Tm is 305 ± 5  °C. There is competition during formation between the eutectic and δ1 phases but δ1 is the only metastable alloy observed. For small size droplets, both the δ1 and eutectic phases show considerable depression of the melting point (size-dependent melting).
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81.16.-c Methods of micro- and nanofabrication and processing
81.30.Fb Solidification
64.70.D- Solid-liquid transitions
81.07.-b Nanoscale materials and structures: fabrication and characterization

Improvement in rectification ratio of an Al-based bulk thermal rectifier working at high temperatures

Tsunehiro Takeuchi, Hiroki Goto, Ryu-suke Nakayama, Yu-ichi Terazawa, Koto Ogawa, Akio Yamamoto, Takashi Itoh, and Masashi Mikami

J. Appl. Phys. 111, 093517 (2012); http://dx.doi.org/10.1063/1.4712420 (7 pages)

Online Publication Date: 7 May 2012

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In this study, we attempted to improve the thermal rectification ratio (TRR = (JAB – JBA)/JBA) of a thermal rectifier consisting of two Al-based alloys, Al72.6Re17.4Si10 and Al71.6Mn17.4Si11, both of which are known to be the 1/1-cubic approximant of icosahedral quasicrystal. In order to estimate the thermal rectification ratio obtainable for a thermal rectifier made from two given materials, we employed a simple method of TRR calculation using the classical Fourier-law of heat conduction. The largest TRR, which exceeded 0.12 for the thermal reservoirs kept at TH = 500 K and TL = 300 K, was experimentally obtained, coinciding with what the calculations predicted. By using the experimentally determined thermal conductivity and the calculation method employed in this study, we show the potential of an Al62Cu25.5Fe12.5 icosahedral quasicrystal as one of the constituent materials of thermal rectifier possessing a TRR magnitude in excess of unity.
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07.20.Ka High-temperature instrumentation; pyrometers

Nanostructure and microripple formation on the surface of sapphire with femtosecond laser pulses

Daejin Kim, Wooyoung Jang, Taehong Kim, Ayoung Moon, Ki-Soo Lim, Myeongkyu Lee, Ik-Bu Sohn, and Sungho Jeong

J. Appl. Phys. 111, 093518 (2012); http://dx.doi.org/10.1063/1.4707951 (4 pages) | Cited 1 time

Online Publication Date: 7 May 2012

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We have demonstrated the creation of polarization-dependent nanogratings with a period of about 250 nm on the surface of sapphire by scanning the femtosecond laser beam with appropriate irradiation conditions. Laser fluence range for nanograting self-formation was very narrow in the slow scan mode. The grating depth variation was observed with atomic force microscope image analysis. To see the composition distribution in the nanostructure, we carried out the Auger signal analysis. In addition, we found a new periodic structure with a period of a few μm and discussed the complex formation mechanism.
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81.16.Rf Micro- and nanoscale pattern formation
42.62.-b Laser applications
42.65.Re Ultrafast processes; optical pulse generation and pulse compression
42.70.-a Optical materials
42.79.Dj Gratings
42.82.Cr Fabrication techniques; lithography, pattern transfer

The mechanism and properties of bio-photon emission and absorption in protein molecules in living systems

Xiao-feng Pang

J. Appl. Phys. 111, 093519 (2012); http://dx.doi.org/10.1063/1.4709420 (14 pages)

Online Publication Date: 7 May 2012

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The mechanism and properties of bio-photon emission and absorption in bio-tissues were studied using Pang’s theory of bio-energy transport, in which the energy spectra of protein molecules are obtained from the discrete dynamic equation. From the energy spectra, it was determined that the protein molecules could both radiate and absorb bio-photons with wavelengths of <3 μm and 5–7 μm, consistent with the energy level transitions of the excitons. These results were consistent with the experimental data; this consisted of infrared absorption data from collagen, bovine serum albumin, the protein-like molecule acetanilide, plasma, and a person’s finger, and the laser-Raman spectra of acidity I-type collagen in the lungs of a mouse, and metabolically active Escherichia coli. We further elucidated the mechanism responsible for the non-thermal biological effects produced by the infrared light absorbed by the bio-tissues, using the above results. No temperature rise was observed; instead, the absorbed infrared light promoted the vibrations of amides as well the transport of the bio-energy from one place to other in the protein molecules, which changed their conformations. These experimental results, therefore, not only confirmed the validity of the mechanism of bio-photon emission, and the newly developed theory of bio-energy transport mentioned above, but also explained the mechanism and properties of the non-thermal biological effects produced by the absorption of infrared light by the living systems.
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87.15.M- Spectra of biomolecules
87.50.wf Biophysical mechanisms of interaction

Molecular beam epitaxy growth of high quality p-doped SnS van der Waals epitaxy on a graphene buffer layer

W. Wang, K. K. Leung, W. K. Fong, S. F. Wang, Y. Y. Hui, S. P. Lau, Z. Chen, L. J. Shi, C. B. Cao, and C. Surya

J. Appl. Phys. 111, 093520 (2012); http://dx.doi.org/10.1063/1.4709732 (8 pages) | Cited 4 times

Online Publication Date: 7 May 2012

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We report on the systematic investigation of optoelectronic properties of tin (IV) sulfide (SnS) van der Waals epitaxies (vdWEs) grown by molecular beam epitaxy (MBE) technique. Energy band simulation using commercial CASTEP code indicates that SnS has an indirect bandgap of size 0.982 eV. Furthermore, our simulation shows that elemental Cu can be used as a p-type dopant for the material. Growth of high quality SnS thin films is accomplished by MBE technique using graphene as the buffer layer. We observed significant reduction in the rocking curve FWHM over the existing published values. Crystallite size in the range of 2–3 μm is observed which is also significantly better than the existing results. Measurement of the absorption coefficient, α, is performed using a Hitachi U-4100 Spectrophotometer system which demonstrate large values of α of the order of 104 cm−1. Sharp cutoff in the values of α, as a function of energy, is observed for the films grown using a graphene buffer layer indicating low concentration of localized states in the bandgap. Cu-doping is achieved by co-evaporation technique. It is demonstrated that the hole concentration of the films can be controlled between 1016 cm−3 and 5 × 1017cm−3 by varying the temperature of the Cu K-cell. Hole mobility as high as 81 cm2V−1s−1 is observed for SnS films on graphene/GaAs(100) substrates. The improvements in the physical properties of the films are attributed to the unique layered structure and chemically saturated bonds at the surface for both SnS and the graphene buffer layer. Consequently, the interaction between the SnS thin films and the graphene buffer layer is dominated by van der Waals force and structural defects at the interface, such as dangling bonds or dislocations, are substantially reduced.
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81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
68.55.ag Semiconductors
71.20.Nr Semiconductor compounds
73.50.Dn Low-field transport and mobility; piezoresistance
73.61.Le Other inorganic semiconductors
78.66.Li Other semiconductors

Conformal growth of Mo/Si multilayers on grating substrates using collimated ion beam sputtering

D. L. Voronov, P. Gawlitza, R. Cambie, S. Dhuey, E. M. Gullikson, T. Warwick, S. Braun, V. V. Yashchuk, and H. A. Padmore

J. Appl. Phys. 111, 093521 (2012); http://dx.doi.org/10.1063/1.4710985 (9 pages) | Cited 1 time

Online Publication Date: 7 May 2012

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Deposition of multilayers on saw-tooth substrates is a key step in the fabrication of multilayer blazed gratings (MBG) for extreme ultraviolet and soft x-rays. Growth of the multilayers can be perturbed by shadowing effects caused by the highly corrugated surface of the substrates, which results in distortion of the multilayer stack structure and degradation of performance of MBGs. To minimize the shadowing effects, we used an ion-beam sputtering machine with a highly collimated atomic flux to deposit Mo/Si multilayers on saw-tooth substrates. The sputtering conditions were optimized by finding a balance between smoothening and roughening processes in order to minimize degradation of the groove profile in the course of deposition and at the same time to keep the interfaces of a multilayer stack smooth enough for high efficiency. An optimal value of energy of 200 eV for sputtering Kr+ ions was found by deposition of test multilayers on flat substrates at a range of ion energies. Two saw-tooth substrates were deposited at energies of 200 eV and 700 eV for the sputtering ions. It was found that reduction of the ion energy improved the blazing performance of the MBG and resulted in a 40% gain in the diffraction efficiency due to better replication of the groove profile by the multilayer. As a result of the optimization performed, an absolute diffraction efficiency of 28.8% was achieved for the 2nd blaze order of the MBG with a groove density of 7350 lines/mm at a wavelength of 13.5 nm. Details of the growth behavior of the multilayers on flat and saw-tooth substrates are discussed in terms of the linear continuous model of film growth.
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68.65.Ac Multilayers
81.15.Cd Deposition by sputtering
81.15.Jj Ion and electron beam-assisted deposition; ion plating
68.55.A- Nucleation and growth
68.35.Ct Interface structure and roughness

Valence-band-ordering of a strain-free bulk ZnO single crystal identified by four-wave-mixing spectroscopy technique

K. Hazu, S. F. Chichibu, S. Adachi, and T. Sota

J. Appl. Phys. 111, 093522 (2012); http://dx.doi.org/10.1063/1.4711103 (6 pages) | Cited 2 times

Online Publication Date: 7 May 2012

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Spectroscopic and temporal four-wave-mixing (FWM) measurements are carried out on a strain-free bulk ZnO single crystal, in order to clarify the valence-band-ordering. Under the collinearly polarized lights with the electric-field component parallel to the c-axis, which can excite dipole-allowed Γ1-excitons, the FWM signal appears only in the energies corresponding to the B-exciton. Under the cross-linear polarization configuration exciting both Γ5- and Γ1-excitons, the FWM signal arising from the two-photon-coherence is absent in the energies corresponding to A-exciton. Both the results indicate that Γ1-exciton state belongs exclusively to B-exciton, meaning that the valence-band ordering is Γ9-Γ7-Γ7 in order of decreasing electron energy for the present strain-free ZnO single crystal.
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71.35.Lk Collective effects (Bose effects, phase space filling, and excitonic phase transitions)

Correlation between the thermodynamic stability of austenite and the shear modulus of polycrystalline steel alloy

H. Terasaki, H. Yamagishi, K. Moriguchi, Y. Tomio, and Y. Komizo

J. Appl. Phys. 111, 093523 (2012); http://dx.doi.org/10.1063/1.4712535 (4 pages) | Cited 2 times

Online Publication Date: 8 May 2012

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Elastic moduli in steel alloys with different martensitic start temperatures were measured during the cooling cycle by using the ultrasonic pulse sing-around method. Our findings show that the shear modulus governs austenite stability prior to martensitic transformation in polycrystalline steel alloys. A non-linear increase in shear modulus is also observed during the cooling cycle. The correlation between elastic modulus and austenite stability in polycrystalline steel alloys is discussed, and the compatibility of the results obtained in this study with the stability theory analyzed in a single crystal is also discussed.
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81.40.Jj Elasticity and anelasticity, stress-strain relations
62.20.de Elastic moduli
64.70.kd Metals and alloys
65.40.G- Other thermodynamical quantities
62.65.+k Acoustical properties of solids
81.30.Kf Martensitic transformations

Evolution of self-assembled type-II ZnTe/ZnSe nanostructures: Structural and electronic properties

S. J. Kim, B.-C. Juang, W. Wang, J. R. Jokisaari, C.-Y. Chen, J. D. Phillips, and X. Q. Pan

J. Appl. Phys. 111, 093524 (2012); http://dx.doi.org/10.1063/1.4705385 (8 pages) | Cited 1 time

Online Publication Date: 9 May 2012

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The strain-mediated evolution of epitaxial ZnTe/ZnSe quantum structures is studied at the atomic scale using spherical aberration-corrected scanning transmission electron microscopy, coupled with electronic properties characterized by photoluminescence spectroscopy. The growth development of these buried quantum dots clearly demonstrates a homogeneous profile with similar pyramidal geometry rather than bi-modal distribution; contradicting prior reports on ZnTe/ZnSe quantum dots. The result is consistent with atomistic theoretical calculations on strain distribution and electronic structure of a modeled quantum dot of similar geometry using a valence force field model. It is also found that the transition from 2-D islands to 3-D quantum dots involves thermally activated carrier transfer process and follows up with formation of extended defects at the quantum dot surface, acting as an effective source for remnant misfit strain relaxation. The new physical understanding concerning the growth of self-assembled ZnTe/ZnSe quantum dots embedded in the active regions provides important information for the measures to control the properties of buried ZnTe quantum dots, setting up a key footstep in developing novel materials of energy conversion.
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81.16.Dn Self-assembly
81.07.Bc Nanocrystalline materials
81.07.Ta Quantum dots
78.55.Et II-VI semiconductors

Thermally activated below-band-gap excitation behind green photoluminescence in ZnO

Kanako Kodama and Takashi Uchino

J. Appl. Phys. 111, 093525 (2012); http://dx.doi.org/10.1063/1.4712624 (9 pages) | Cited 3 times

Online Publication Date: 9 May 2012

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We explore the temperature dependence of the photoluminescence (PL) and photoluminescence excitation characteristics of the green emission band in undoped ZnO crystals. We find that there exists a thermally assisted luminescence channel, which tends to dominate over the existing luminescence channel especially under below-band-gap excitation. Shallow donor electrons are likely to contribute to the thermally assisted luminescence process by being thermally excited from the donor levels to the conduction band. We hence suggest that the shallow donor centers, which are presumably attributed to zinc interstitials and will not act as emitting centers by themselves, are indirectly responsible for the green PL emission. The most probable candidate of the emitting center is the neutral oxygen vacancy, showing the green emission either via a direct internal emission process or an indirect thermally assisted emission process in combination with shallow donor centers.
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78.55.Et II-VI semiconductors
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
71.55.Gs II-VI semiconductors
61.72.jd Vacancies
61.72.jj Interstitials
71.20.Nr Semiconductor compounds
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